The present invention relates to an anti-blocking system for a road vehicle equipped with a hydraulic multi-brake installation in which a pressure modulator is coordinated to each of the wheel brakes adapted to be subjected to the regulation which has an output pressure space connected to the respective wheel brake, by way of which a brake pressure produced by means of the brake device is adapted to be coupled into the wheel brake cylinder, respectively, cylinders of this wheel brake.
An anti-blocking system of this type is the subject of the non-pre-published German Patent Application DE 35 31 137 Cl.
In the anti-blocking system described therein for a vehicle with four-wheel drive and with a brake installation having front axle-/rear-axle brake circuit division, which is combined with a propulsion-regulating arrangement, a tandem-master cylinder adapted to be actuated by way of a brake force-servo unit of customary type of construction is provided for the actuation of the brake installation, which includes a pressure outlet coordinated to the rear axle brake circuit and a pressure outlet coordinated to the front axle brake circuit. A pressure modulator of its own is coordinated to each of the wheel brakes which according to construction and function is analogous to a master cylinder for a single-circuit brake installation, that is, has still a single outlet pressure space with which the respective wheel brake is connected. For the actuation of the pressure modulator in the sense of a brake pressure build-up, the output pressure of the tandem-master cylinder is coupled to an inlet-pressure space of the pressure modulator, which is delimited pressure-tight by a piston flange against a follower space, which is in communicating connection with the brake fluid reservoir container of the brake apparatus and in its turn is delimited against the output pressure space by a second flange, which is delimited housing-fixedly by a partition wall of the modulator housing, through which a plunger-shaped extension of the modulator piston passes and is sealed off against the same. A piston movably delimiting a control pressure space engages at the end of this plunger-shaped extension, by the pressure actuation of which with the output pressure of an auxiliary pressure source, the modulator piston is adapted to be pushed back against the brake pressure established in its inlet space, as a result of which a volume enlargement of the outlet pressure space and thereby a pressure reduction-phase of the anti-blocking regulation is attainable. Pressure-maintaining phases are adapted to be controlled in that the control pressure space is adapted to be closed up against the auxiliary pressure source--valve-controlled--, and in that also the wheel brakes are adapted to be closed off, valve-controlled, against the pressure outlet of the pressure modulator.
The construction of the pressure modulator, explained so far, which is required for the realization of the anti-blocking regulation, requires already a considerable structural length which corresponds at least to that of a tandem master cylinder of usual construction and is further increased in that for the realization of the propulsion-regulation function a further actuating pressure space must be provided, by the pressure actuation of which--also valve-controlled--the piston is displaceable in the outlet pressure space of the pressure modulator in the sense of a pressure build-up also without actuation of the brake device. For the required sealing of the piston against the housing of the modulator, at least four slide seals are required, even if the modulator is designed only for the anti-blocking regulation, and, if the pressure modulator must also be constructed for the realization of a propulsion regulation, at least two further annular seals become necessary whose friction must be overcome also during a normal braking.
An anti-blocking system realized in this manner therefore entails at least the following disadvantages:
1. Large space requirement of the pressure modulators,
2. Relatively high susceptibility to trouble because of numerous annular seals prone to wear, and
3. High friction losses which are conditioned by the annular seals and which may lead to the fact that the brake installation requires disproportionately high actuating forces in case of a failure of the brake force servo unit so that the brake installation under unfavorable conditions is still hardly actuatable and/or a failure of the brake installation altogether is misleadingly indicated to the driver.
The anti-blocking system of a similar type disclosed in the German Offenlegungsschrift DE 33 39 285Al entails essentially the same disadvantages, in which pressure modulators individually coordinated to the wheel brakes are constructed as 1/1-pressure converters, whereby, corresponding to the front axle/rear axle brake circuit division, the pressure modulators coordinated to the two front wheel brakes and the pressure modulators coordinated to the two rear wheel brakes are combined into a pressure modulator cylinder each by means of a common housing. These pressure modulators have each a control pressure space and an outlet pressure space which are delimited with respect to one another housing-fixedly by a partition wall and are movably limited by one piston flange each, which are securely connected with each other by a piston rod that displaceably passes pressure-tight through a central bore of the partition wall. The outlet pressure spaces, with which one of the wheel brakes each is connected, are thereby arranged in the mutually remotely arranged end sections of the cylinder bore each accommodating two pressure modulators, which are each closed off pressure-tight by a plug. The piston flanges of the modulator pistons movably limiting the control pressure spaces are displaceably guided pressure-tight in a central section of the cylinder bore extending between the two partition walls and form at the same time boundaries movable in the axial direction of an anti-blocking control pressure space extending between the same, which is adapted to be alternately connected by way of an outlet valve and an inlet valve with the pressureless tank, respectively, the high pressure output of an auxiliary pressure space provided for the anti-blocking control. The control pressure spaces of the pressure modulators provided for the front wheel brakes are connected to one pressure outlet each of a brake device constructed as tandem master cylinder. The control pressure spaces of the pressure modulators coordinated to the rear wheel brakes are connected by way of one proportional valve each to the control pressure space of the pressure modulator which is coordinated to the front wheel brake of the same vehicle side so that an H-brake circuit division is provided as regards the brake actuation. The purpose of these proportional valves is thereby to keep the brake pressure increase at the rear wheel brakes smaller than at the front wheel brakes during a normal braking, i.e., during a braking not subjected to the regulation, in order to take into consideration in this manner the dynamic unloading of the rear axle and to anticipate a blocking tendency at the rear axle. During a normal braking, the modulator pistons experience--in the brake-pressure-build-up phase--a displacement in the sense of an increase of their control pressure spaces and a corresponding reduction of their outlet pressure spaces whereby the anti-blocking control pressure spaces extending between two pressure modulators are relieved in the direction to the tank of the auxiliary pressure source by way of their outlet valves disposed in the open position. If a blocking tendency occurs at the front and/or the rear axle, then the outlet valve of the respective anti-blocking control pressure space is activated into its closing position and the inlet valve, closed up to then, into the open position thereof, and as a result thereof, the high output pressure of the auxiliary pressure space is coupled into the respective anti-blocking control pressure space, which has as a consequence that the modulator pistons of both pressure modulators coordinated to the front wheel brakes and/or to the rear wheel brakes move "apart", that is, in the sense of an increase of their outlet pressure spaces and therewith in the sense of a reduction of the brake pressure in the connected wheel brakes. The activation of the inlet and outlet valves is thereby--meaningfully--so made that the anti-blocking regulation at the front axle operates according to the so-called "Select High" principle, that is, the regulation of the front axle starts when a blocking tendency occurs at both front wheels, and takes place at the rear axle according to the "Select Low" principle, that is, a pressure actuation of the anti-blocking control pressure spaces of the pressure modulators coordinated to the rear wheel brakes, producing the brake pressure reduction at both rear wheel brakes, takes place "already" when a blocking tendency occurs exclusively at one of the two rear wheels of the vehicle. As regards the regulation, a front axle-/rear axle brake circuit division is thus selected in the known anti-blocking system.
The two piston flanges of the modulator pistons are displaceably sealed against the bores delimiting the control pressure space and the outlet pressure space in the radial direction by means of one annular seal fixedly arranged at the piston. The piston rod connecting with each other the two piston flanges of the pressure modulators, is sealed off against the partition wall bore by means of a housing-fixedly arranged annular seal. The annular seals fixed at the piston are thereby constructed as sealing sleeves whose outer sealing lips are pressed radially outwardly by the pressure prevailing in the space under consideration in good sealing abutment with the respective bore wall. Such a sealing sleeve is also arranged at the piston flange offsetting the control pressure space of the pressure modulator against the anti-blocking control pressure space on the side facing the anti-blocking control pressure space in order to reliably preclude that during the anti-blocking regulation operation pressure medium can flow over from the anti-blocking control pressure space into the brake pressure control pressure space of the respective pressure modulator which would impair the function of the anti-blocking regulation. Four annular seals per modulator are thus provided also with the pressure modulators of this further known ABS which entail considerable friction losses. Owing to the pairwise symmetrical arrangement, as viewed in the axial direction, of two pressure modulators each with respect to the transverse center plane of the anti-blocking control pressure space extending perpendicularly to the central axis, an unfavorably great structural length results for the pressure modulation cylinders which further considerably exceeds that of a customary tandem-master cylinder. The know anti-blocking system thus entails at least the disadvantages as regards the space requirement of its hydraulic unit as the anti-blocking system according to the DE 35 31 157C1.
The object of the invention is therefore to improve an anti-blocking system of the aforementioned type to the effect that it can be realized, also in the case of a combination with a propulsion-regulation installation with considerably smaller technical expenditure, especially considerably more simple construction of the pressure modulators and smaller space requirement thereof, possesses a considerably smaller trouble proneness, and therebeyond assures that also in case of a failure of the brake force servo unit, the pedal-actuating force required for achieving a still adequate braking deceleration remains within acceptable limits.
The underlying problems are solved according to the present invention by the features to be described more fully hereinafter.
The pressure modulators provided in accordance with the present invention as brake pressure-adjusting members for the anti-blocking regulation, which in the pressure-reduction phases act like an intermediate reservoir for brake fluid drained out of the wheel brakes subjected to the regulation and in the pressure build-up phases, analogous to a single-circuit master cylinder, respectively, to a pressure converter, as long as the outlet pressure space of the respective pressure modulator is closed off against the brake device, are extraordinarily simple as regards construction and are realizable with small space requirement which enables altogether a compact construction of the hydraulic unit of the anti-blocking system. As the stepped or differential pistons providing the movable boundaries of the control pressure spaces of the pressure modulators and their outlet pressure spaces remain "stationary" in their base positions during a normal braking, i.e., a braking not subjected to the regulation, whereby the output pressure space of the respective pressure modulator acts exclusively like a section of constant cross section of the brake line branch leading from the master brake line to the respectively connected wheel brake, on the one hand, the wear of the seals provided within the frame of the pressure modulators is reduced to a minimum and, on the other, it is also avoided that in case of a failure of the brake force servo unit, disproportionately high actuating forces become necessary in order to attain a still adequate minimum deceleration of the vehicle.
According to another feature of the present invention, a preferred dimensioning of the auxiliary source which is favorable for the functioning safety of the anti-blocking system according to the present invention, whose outlet pressure is adapted to be coupled into the control pressure spaces of the pressure modulators, is obtainable if the minimum amount, by which the outlet pressure P.sub.A of the auxiliary pressure source is higher than the maximum attainable outlet pressure of the brake device multiplied with the ratio of the areas of the differential pistons of the pressure modulator, has a value between 30 bar and 60 bar and preferably has a value of about 50 bar.
The hydraulic separation resulting from the construction of the pressure modulators provided according to the present invention of the function-control circuit which leads from the auxiliary pressure source by way of the function control valve to the control pressure space movably delimited by the smaller piston step of the respective pressure modulator, from the brake pressure-control circuit providing the brake pressure coupling into the respective wheel brake, which leads by way of the outlet pressure of the respective pressure modulator changeable as regards its volume, enables without difficulty to utilize a different working medium in the function-control circuit than the brake fluid used in the brake pressure supply circuits. However, it is preferred to use brake fluid as working medium also in the control circuit. A leakage oil space between the control pressure space and the outlet pressure space of the pressure modulator which is otherwise necessary and is to be kept pressureless, can be dispensed with thereby, and a single, preferably housing-fixedly arranged annular seal suffices in order to seal off the control pressure space against the outlet pressure space of the pressure modulator. Both the number of the wear-prone seals is minimized thereby as also structural length is "economized."
The pressure modulators of a brake circuit including two wheel brakes may thereby be combined without difficulty into a structural unit, appropriately the pressure modulators of those wheel brakes which have the same displacement volume, that is, the pressure modulators of the rear wheel brakes, on the one hand, and the pressure modulators of the front wheel brakes on the other.
An operationally reliable, simple construction of the pressure modulators can be achieved in that a central bore step is arranged between the bore step accommodating the smaller piston step and the bore step accommodating the larger piston step, whereby the diameter of the central bore step has a value between the diameters of the larger and of the smaller piston step, and in which the pressure outlet channel, by way of which the wheel brake cylinder is connected to the respective pressure modulator outlet pressure space, terminates in the outlet pressure space inside of this central bore step.
According to another feature of the present invention, both the electronic technical control expenditure as also the space requirement necessary for the pressure inlet-control valves can be reduced by the use of hydraulically actuatable pressure inlet-control valves.
According to still another feature of the present invention, additional safety is achieved by the use of hydraulic auxiliary control circuit that the pressure-inlet control valves and/or the function-control valves reach with the start of a braking the base position to be assumed at least initially. It is understood thereby that these auxiliary control circuit must be designed sufficiently "weak" in order that the electromagnetic or hydraulic control circuits, by means of which the pressure-inlet control valves and/or the function-control valves are activated into their regulating correct functioning positions, remain dominant.
According to a still further feature of the present invention, it can be effectively precluded by a safety installation of simple construction that a malfunction of the auxiliary pressure source may lead during a response of the anti-blocking regulation to an "emptying regulating" of the brake circuit, at which the regulation has become effective and may lead in the extreme case to the failure of the brake installation.
According to another feature of the present invention, an effective propulsion regulation installation (ASR) can be realized in accordance with the present invention by means of additional function elements which are linked with only a slight technical additional expenditure compared to the anti-blocking system of the present invention, under utilization of all function elements of the anti-blocking system, whereby the propulsion regulation installation can be designed both for a vehicle with single-axle drive as also for a vehicle with four-wheel drive.
According to still another feature of the present invention, appropriate control functions, for example, a stepwise brake pressure build-up and reduction in the brake circuit of the driven vehicle wheels, can be controlled in a simple manner by a propulsion-regulation-control valve in accordance with the teachings of the present invention.
Such a control valve also offers the possibility of the activation, advantageous for safety reasons, by means of which it is avoided that an anti-blocking regulation operation starting immediately after a propulsion regulation phase may lead to an emptying regulating of the brake circuit subjected to the regulation in the outlet pressure space of the pressure reducer provided for the propulsion regulation.
In a combined realization of an anti-blocking and of a propulsion regulation, it is particularly advantageous if, according to another feature of the present invention, both the pressure inlet-control valves as also the function-control valves are constructed as electrically controllable valves.
By contrast, according to still another feature of the present invention, the two position valve which provides for closing the main brake line of a brake circuit adapted to be actuated with brake pressure in the course of the propulsion regulation against the coordinated outlet of the brake device, can be constructed as hydraulically controlled valve, whereby the inlet pressure of the pressure reducer provided within the frame of the propulsion regulation installation can be utilized for the shifting control of the two-position valve.